Battery backup runtime: estimate hours from Wh and device watts
Estimate how long a power bank or backup battery can run a device after usable capacity and conversion losses.
How to calculate it
Nominal battery watt-hours are an energy quantity. A first estimate is runtime hours = usable Wh ÷ device average watts. Usable Wh may be lower than the rating because the battery is not fully discharged or because an inverter or USB converter wastes some energy. If you have a conversion efficiency estimate, usable output Wh = rated Wh × usable fraction × efficiency. Keep instantaneous startup watts separate from average operating watts; the inverter must still support peak load.
Worked example
A battery rated 600 Wh with 80% permitted use and 85% inverter efficiency provides approximately 600 × 0.8 × 0.85 = 408 Wh to the load. A device averaging 60 W might run 408 ÷ 60 = 6.8 hours. A 300 W appliance on the same battery would get roughly 1.36 hours under the simplified model, if the inverter can supply its startup power. Neither result is a manufacturer guarantee.
What can change the result
Battery age, temperature, cutoff voltage and high discharge current can reduce usable energy. A device may cycle between active and standby, so use measured average watts for runtime. Some capacity labels use mAh at a particular cell voltage; mAh cannot be compared across voltages without conversion to Wh. Plan a reserve for essential equipment and follow manufacturer safety limits.
Use the calculators and keep reading
Use the battery runtime calculator with capacity, device load and efficiency assumptions. Check the electrical power calculator if you have voltage and current instead of watts. The related standby-power guide helps measure an idle load that continues draining the battery.
Try the battery runtime, electrical power, electricity calculator and read this related article for the context and assumptions.